On the trainer you tend to sit in one position for a longer time than if you were outside, it makes it more uncomfortable. You also never get a chance to coast.

Our LBS has computrainer set up with 4 bikes. It's free for club members. I do that once a week. It really helps having friends to ride with. We usually do a 25-30 mile course.

Starting next week I'll be doing group trainer ride with another club taught by one of the local pro's. That should be fun (more fun than riding alone in my basement anyways).

My last ride of the week (I try to get 3/week), I watch a movie and pedal easy. I'm trying to increase my time on the bike. I have yet to do a 2 hr trainer ride.